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Current/voltage limiting of a large array of LEDs ... without LED driver ICs?

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I wanted a rude username:
Let's say you want to run an 8 x 8 array of LEDs from a microcontroller (either directly attached rows and columns, or using shift registers), and you want the brightness to remain more or less constant at any voltage between 3 V and 5 V. How would you do it without using expensive LED drivers which provide constant current outputs?

PWM would work and is easy, but LEDs are more efficient when run at 100% duty cycle at a lower voltage.

fcb:
Have your microcontroller monitor it’s supply voltage and then adjust the duty cycle to suit brightness requirement.

If you are directly driving the matrix from the microcontroller pins then you’ll probably want some resistors in-line (unless you can rely on the impedance of the microcontroller output driver).

Depending on your scheme (8x, 64x) your adjustment could be a simple LUT, 8 LUTs or some polynomial expression.

I wanted a rude username:
Those are all solid suggestions which I've used on other projects, but was hoping for a means to control brightness via current or voltage instead of PWM.

Leading idea so far is having the shift register powered by a buck converter with a fairly precise voltage. That should be more efficient than current-limiting resistors and/or PWM, but seems to risk uneven lighting due to variations even within binned LEDs ...

fcb:
As we don't know much about your application (other than it needs to be cheap and power efficient), it's difficult to suggest to much more.

Is size an issue (!) and what are you displaying - video? What colour(s) of LED? My experience of LED's is that temperature will have more of an effect on Vfd than manufacturing variations.

Either way - you will probably want some sort of current control scheme if you need even brightness.

mariush:

--- Quote from: I wanted a rude username on February 05, 2020, 07:27:44 am ---Let's say you want to run an 8 x 8 array of LEDs from a microcontroller (either directly attached rows and columns, or using shift registers), and you want the brightness to remain more or less constant at any voltage between 3 V and 5 V. How would you do it without using expensive LED drivers which provide constant current outputs?

PWM would work and is easy, but LEDs are more efficient when run at 100% duty cycle at a lower voltage.

--- End quote ---

You'll have to define what expensive means to you.
A led driver can be as cheap as 3 cents.

Here's for example a 3 cents 8 channel led driver which works like a shift register (you'll find these from multiple companies, but this one's available in 220k units, so you could buy a few thousands if you want): https://lcsc.com/product-detail/LED-Drivers_Shenzhen-Sunmoon-Micro-SM74HC595D_C93838.html
You can either use one for each row (so around 25 cents for everything) or you could use microcontroller pins to send power to one row at a time (or use micro to turn on a transistor which sends power to a particular row)

For 10 cents, you can get TM150, a chip designed to drive 4 seven segment + dot  LED digits, but which can be rearranged into 4 x 8 matrix (see page 8 of datasheet) : https://lcsc.com/product-detail/LED-Drivers_TM-Shenzhen-Titan-Micro-Elec-TM1650_C44444.html
So 2 of these would be enough.
 
27 cents (@100pcs) gets you a 16 x (seven segment + dot) led driver : https://lcsc.com/product-detail/LED-Drivers_TM-Shenzhen-Titan-Micro-Elec-TA6932_C113719.html

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